FIG. 6(a) is a cross-sectional view illustrating a prior art polycrystalline Si solar cell, for example, as shown in "20TH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE 1988 Technical Digest pp1600-1603", and FIG. 6(b) is a conceptual view of hydrogen passivation of the polycrystalline Si solar cell. In the figure, reference numeral 1 designates a p type polycrystalline Si layer. An n type diffusion layer 2 is formed on a light receiving surface of the p type polycrystalline Si layer 1. A high dopant impurity concentration p type layer 3 is disposed on a rear surface of the p type polycrystalline Si layer 1. An anti-reflection film 4 is disposed on an upper surface of the n type diffusion layer 2. A surface electrode 5 is located at apertures in the anti-reflection film 4 which is patterned on the upper surface of the n type diffusion layer 2. A lattice shaped rear surface electrode 6 is located on a rear surface of the high concentration p type layer 3. Reference numeral 7 designates hydrogen ions irradiating the rear surface of the polycrystalline Si solar cell.
A description is given of a method for producing a polycrystalline Si solar cell employing hydrogen passivation.
Thermal diffusion of phosphorus into the light receiving surface of the p type polycrystalline Si layer 1 forms n type diffusion layer 2 about 400 .mu.m thick and junction at an interface with the Si layer 1. Subsequently, Al paste is applied to the rear surface of the p type polycrystalline Si layer 1 by screen printing and it is sintered, whereby a high dopant impurity concentration p type layer 3 is produced. The excess Al paste is removed with an acid etchant.
Thereafter, the surface electrode 5 is produced on the n type diffusion layer 2 by sintering an Ag paste formed by screen printing, and the lattice-shaped rear surface electrode 6 is formed on the rear surface by sintering an Ag paste formed by screen printing. Titanium dioxide (TiO.sub.2) is deposited by atmospheric pressure CVD on the n type diffusion layer 2 at the light receiving surface, whereby the anti-reflection film 4 is produced. Finally, hydrogen ions 7 are implanted into the rear surface of the polycrystalline Si solar cell that is formed as described above in an ion implantation apparatus having an acceleration energy of 2 .about.10 KeV, thereby achieving hydrogen passivation. Hydrogen is engaged with dangling bonds of the semiconductor, i.e., the polycrystalline Si layer 1, by hydrogen passivation, whereby the crystal grain boundaries and surface defects of the polycrystalline Si layer 1 are electrically inactivated, and the energy conversion efficiency of the solar cell is improved from 14.4% to 15.2%.
In the prior art method for producing a solar cell as described above, the hydrogen passivation penetrates to only one hundred .mu.m from the rear surface of the solar cell because the polycrystalline Si layer is thick, and the effect of hydrogen passivation does not sufficiently reach the depletion layer at the junction. Thus, the improvement in the energy conversion efficiency only amounts to, at most 7-8% even in a solar cell having an energy conversion efficiency larger than 10% before the hydrogen passivation. In addition, because the hydrogen passivation is performed after forming a rear surface electrode, the hydrogen passivation is not effected at more than 20% of the entire rear surface area of the polycrystalline Si layer on which portion the rear surface electrode is formed.